Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

I signed It!

CLAs look good, thanks!

It should still be "optional" in the Maven sense (meaning that it needs to be present when compiling Guava but not when compiling projects that use Guava. What problem are you seeing?

Ok. I'm closing this PR so I can do more investigation. When we updated to version 20.0 we started getting

Error:java: cannot access com.google.errorprone.annotations.CanIgnoreReturnValue
  class file for com.google.errorprone.annotations.CanIgnoreReturnValue not found

when compiling the project in IntelliJ. The workaround is to explicitly add the error_prone_annotations dependency to our app but I'm not sure what part of our code is trying to use that annotation.